The subject matter of the present application is related to the subject matter of U.S. patent application Ser. No. 08/775,466 entitled xe2x80x9cMethod and Apparatus for Providing High Speed Services Using a Wireless Communications Systemxe2x80x9d to Thomas K. Fong, Paul Shala Henry, Kin K. Leung, xiaoxin Qiu, Nemmara K. Shankaranarayanan and assigned to ATandT Corp., filed on Dec. 30, 1996, U.S. patent application Ser. No. 08/832,546 entitled xe2x80x9cMethod and Apparatus for Resource Assignment in a Wireless Communications Systemxe2x80x9d to Xiaoxin Qiu and Kapil Chawla, filed Apr. 3, 1997 and U.S. patent application Ser. No. 08/982,510 entitled xe2x80x9cDynamic Resource Allocation Method and Apparatus for Broadband Services in a Wireless Communications Systemxe2x80x9d to Kin K. Leung and Arty Srivastava, filed Dec. 2, 1997, the entire disclosures of which are hereby incorporated by reference.
The invention relates to wireless communications systems. More particularly, the invention relates to a method and apparatus for sector based resource allocation in a broadband wireless communications system.
The need for high-speed broadband packet services will grow tremendously as telecommuting and Internet access become increasingly popular. Customers will expect high quality, reliable access to high-speed communications from homes and small businesses in order to access, for example: (a) the World Wide Web for information and entertainment; (b) office equipment and data from home at rates comparable to Local Area Networks (LANs); and (c) multimedia services such as voice, image and video. Although varying with application, effective broadband communication requires a bandwidth sufficient to permit a data rate up to the range of several tens of Mega-bits per second (Mbps).
Traditional wireless communications systems have a problem delivering high-speed services because of the amount of bandwidth these services require. Bandwidth is a key limiting factor in determining the amount of information that a system can transmit to a user at any one time. The concept of bandwidth may be better understood using an analogy. If information carried by a network were water, and links between communication sites were pipes, the amount of water (i.e., information) a network could transmit from one site to another site would be limited by the diameter of the pipes carrying the water. The larger the diameter of the pipe, the more water (i.e., information) can be transmitted from one site to another in a given time interval. Likewise, the more bandwidth a communications system has available to it, the more information it can carry.
Traditional wired communications systems using modems and a physical transmission medium, such as twisted pair copper wire, cannot currently achieve the data rates necessary to deliver high-speed service due to bandwidth limitations (i.e., small pipes). Promising technologies for xe2x80x9cbroadbandxe2x80x9d (i.e., large pipes) access include the Asymmetrical Digital Subscriber Loop (ADSL) and Hybrid Fiber-Coax (HFC). These wired-network approaches to providing high-speed access, however, could be expensive and time consuming to install.
The benefit of wireless systems for delivering high-speed services is that they can be deployed rapidly without installation of local wired distribution networks. However, traditional wireless systems such as narrowband cellular and Personal Communications Services (PCS) are bandwidth limited (small pipes) as well. As an alternative, wireless solutions such as Multichannel Multipoint Distribution Service (MMDS) and Local Multichannel Distribution Service (LMDS) have become attractive but these solutions presently offer limited uplink channel capacity. Moreover, these solutions in their current system design may not be capable of supporting a large number of users.
One solution for solving the bandwidth limitation problem for wireless systems is to maximize the available bandwidth through frequency reuse. Frequency reuse refers to reusing a common frequency band in different area, or xe2x80x9ccells,xe2x80x9d within the system. Refer, for example, to FIG. 1 which shows a typical wireless communication system. A Base Station (BS) 20 communicates with several Terminal Stations (TS) 22. The BS 20 is usually connected to a fixed network 24, such as the Public Switched Telephone Network (PSTN) or the Internet. The BS 20 could also be connected to other base stations, or a Mobile Telephone Switching Office (MTSO) in the case of a mobile system. Each TS 22 can be either fixed or mobile.
The BS 20 communicates information to each TS 22 using radio signals transmitted over a range of carrier frequencies. Frequencies represent a finite natural resource, and are in extremely high demand. Moreover, frequencies are heavily regulated by the government. Consequently, each cellular system has access to a very limited number of frequencies. Accordingly, wireless systems attempt to reuse frequencies in as many cells within the system as a possible. To accomplish this, frequency reuse patterns are designed for cellular systems. A major factor in designing a frequency reuse pattern is the attempt to maximize system capacity while maintaining an acceptable Signal-to-Interference Ratio (SIR) for correct signal detection. SIR refers to the ratio of the level of the received desired signal to the level of the received undesired signal.
To achieve frequency reuse, a cellular system takes the total frequency spectrum allotted to the system and divides it into a set of smaller frequency bands. The geographic area covered by a cellular communications system is organized into cells and/or sectors, with each cell typically containing a plurality of communications sites, such as a BS 20 and TS 22. The cells can be any number of shapes, such as a hexagon, and groups of cells can be formed with each cell in the group employing a different frequency band. These groups can be repeated until the entire service area is covered. Thus, in essence, the frequency reuse pattern determines the distance between cells that use common frequency bands. The goal of a pattern is to keep interference due to the common use of the same frequency band in different cells, or xe2x80x9cco-channelxe2x80x9d interference, below a given threshold to ensure successful signal reception. The advantage of this frequency reuse plan to manage co-channel interference can also be achieved by a time domain approach. In such an approach, the whole frequency spectrum is used for each transfer, but time is divided into frames, each of which consists of multiple frames. Different frames are reused in various cells in a way similar to frequency reuse patterns, as discussed above. Thus, there is a direct analogy between frequency bands and time frames.
Although the reuse of bandwidth in cellular systems is limited by this co-channel interference, directional antennas at both the BS and the TS in fixed wireless systems can help reduce the amount of interference from neighboring sectors and cells. U.S. patent application Ser. No. 08/775,466 discloses a Staggered Resource Allocation (SRA) method which uses a distributed, dynamic resource allocation algorithm for fixed wireless networks where the same spectrum is shared by every sector and cell on a dynamic, time basis. Relying on directional antennas to suppress interference, the algorithm schedules concurrent packet transmissions in various sectors and cells that cause little interference to each other, while sectors causing major interference to each other do not transmit simultaneously. There is a specific sequence in which sectors are labeled, and a specific schedule in which time sub-frames are used in each sector. As a result, the SRA scheme can yield a throughput in excess of 30% per sector for typical scenarios.
In such systems, a small fraction of terminals, typically near a cell boundary, will experience shadow fading conditions where interference cannot be adequately suppressed by the finite Front-to-Back (FTB) gain ratios of the terminal antennas. Techniques are available to increase the cell coverage to include such xe2x80x9cvulnerablexe2x80x9d terminals. For example, U.S. patent application Ser. No. 08/832,546 discloses a Time Slot Reuse Partitioning (TSRP) method which uses multiple time domain reuse patterns which provide different SIR characteristics. Terminals are categorized based on SIR requirements and assigned to different reuse patterns, i.e. they use time slots allocated to the respective patterns. A time frame is divided into portions that are xe2x80x9csharedxe2x80x9d in a cell and xe2x80x9cdedicatedxe2x80x9d portions that are not shared. The dedicated portions are used for the vulnerable terminals to ensure an adequate SIR. An Enhanced SRA (ESRA) scheme, such as the one described in U.S. patent application Ser. No. 08/982,510 can augment the sector labeling scheme and modify the SRA transmission schedule to protect the vulnerable terminals.
It is known that a practical directional antenna used at a base station to cover a sector (referred to as a sector antenna) will have a radiation pattern that overlaps with those of adjacent sectors. A representative directional antenna pattern, and the sector 202 that it covers, is shown in FIG. 2. If the same frequency were used simultaneously in adjacent sectors 204, 206, it is likely that a user terminal would receive strong interference from adjacent sectors for the downlink. Similarly for the uplink, base station antennas of adjacent sectors can receive strong interference from terminals located at the sector boundary. With practical directional antennas, in most cases, no two adjacent sectors should use the same channel. Thus, the SRA method, developed with an ideal antenna pattern assumption, faces performance limitations.
Another problem with known techniques such as the SRA, ESRA and TSRP methods is that they cannot easily handle an increase in the number of users or the amount of traffic after the base stations are set up and terminal antennas are adjusted to point to their associated base stations. Besides the obvious option of using more spectrum, higher capacity is usually achieved by increasing the number of cells or the number of sectors per cell. The common practice of increasing the number of cells, or xe2x80x9ccell splitting,xe2x80x9d for capacity growth is not a good option for fixed wireless networks because the existing directional terminal antennas in the new cell must be re-oriented to a new basexe2x80x94a huge and undesirable task. Moreover, sector splitting will be very involved, such as changing the time-slot assignment sequence for all sectors in the SRA and ESRA methods or re-evaluating the SIR characteristics of all terminals in the TSRP method with additional new sectors.
Finally, a sector labeling plan and time reuse pattern should be robust enough to allow for the inevitable deviations from a regular cell layout. Due to terrain variations and differences in the volume of traffic expected in an area, cells may not be of exactly equal size and shape. Similarly, the number and size of sectors may not always be the same in each cell. Such deviations are not easily achieved with known techniques.
In view of the foregoing, it can be appreciated that a substantial need exists for a method and apparatus for resource allocation in a broadband wireless communications system that accounts for realistic antenna patterns, capacity growth, non-uniform traffic density and irregular cell layout, and solving the other problems discussed above.
The disadvantages of the art are alleviated to a great extent by the method and apparatus for resource allocation in a broadband wireless communications system. A service region is divided into a plurality of cells, and each cell is divided into a plurality of labeled sectors. Each label is selected to avoid an unacceptable amount of interference from any other sector while ensuring that at least two sectors in a cell share the same label. Transmissions are scheduled for a cell by assigning each communications site a channel, such as a time slot, associated with the sector""s label. The transmissions are then communicated according to this schedule.
With these and other advantages and features of the invention that will become hereinafter apparent, the nature of the invention may be more clearly understood by reference to the following detailed description of the invention, the appended claims and to the several drawings attached herein.